The power outage at Partizan Belgrade's stadium even affected the away dressing room for Arsenal.
After one of the floodlights failed following a series of power cuts in the main stand, there was some doubt whether the match would actually go ahead before the referee got agreement from both sides.
Sagna revealed the Arsenal players saw the funny side of things, but got serious as soon as they went on to the pitch.
"The lights went off, so we had to use the lights from the phones to get ready," he said.
"It was quite funny, but we stayed focused and came on the pitch to get the win."